TRK12/120 Restoration

[Steve McVoy]

Aquadag is useless in prewar TV sets. The purpose is to act as a capacitor to smooth out the 15 kHz from the flyback in postwar sets. Prewar sets used transformer power supplies with 60 Hz, so all the filtering is done in the power supplies with capacitors.

[dtvmcdonald]

Yes, I knew aquadag was useless for such a set. But I had recalled somebody
saying it was there.

Incidentally Steve, I found the CRT base pin assignment, it was on
a paper formerly glued to the base but now loose.

I looked at the alignment: the video IF as amazing: it looks like the CT-100
its so good. The RF was not so good: on neither Channel 3 nor 4
was the audio really in range of the fine tuning though you could
hear sound. And 3 and 4 were off in opposite directions. And the
correct spot was out of range on the adjustments by a lot. I got Ch. 4
OK by a slight bend of the strap but Ch. 3 required that I solder
a copper strip 1/4 inch wide to the tuning loop to get the inductance
down a bit. The tuning adjustments for the antenna coils were
well within range and were quite a bit off of flattest response.

Oh yes: the audio carrier ended up at 8.11 MHz.

[dtvmcdonald]

Today's progress: spent over three hours standing at a lathe
making a special HV connector to attach the cup connector
to the old connector. It been 15 years since I did extensive
lathe work, but I still remember it all.

Oh yes ... did I mention that I didn't take shop class
in high school ... rather, at Harvard. Yes, we had a noncredit
shop class in the chem department!

Tonight I attached the yoke and plugged in the H and V output tubes and the damper, and turned it on. It worked OK and nothing bad happened. I'm
using a horizontal output tube ostentatiously labeled "Fender" and not ostentatiously labeled "made in Russia".

[Steve McVoy]

RCA specified a special version of the 6L6 for that set. It is metal. If I remember correctly, you can identify the proper one by a double metal ring at the bottom rather than a single one. I have no idea how modern tubes would work.

[dtvmcdonald]

I got a picture on the thing tonight.
The sweeps seem to still be OK. The cathode and
grid signals to the CRT seem reasonable ...
say +75 on the cathode and -5 to +25 to +50 on the grid.

However the CRT G2 and focus seems totally out of whack.
With those K and G1 voltages I get normal brightness but very very
bad focus. If I either increase the video drive or decrease the
cathode voltage ... making it excessively bright ... I get good
focus but its very unstable.

I set the resistor chain on the HV to the ratios given by the
schematic ... the set came with the G2 set way up the
chain from what the schematic says.


Does anybody with one of these radar tubes know what the
correct G2 and focus voltages are?

Doug McDonald

[Steve McVoy]

The only difference between the radar tube and the 12AP4 voltages is the filament, which is 6.3 rather than 2.0. Focus and G2 are the same: about 250v for G2 and 2000v for focus.

[dtvmcdonald]

Let me get those voltages for sure right. They are referenced to the
cathode, yes???? So with the cathode at +75 the G2 should be at
75+250 = 325 and the focus should be adjustable around 2075, with
respect to ground.

The instability is perhaps due to bad resistors in the bleeder chain.
I will replace them today.

Close examination of the HV cable showed it to have cracked
rubber insulation. It was replaced.

I forgot to mention that the grid lead to the CRT was dead.
It used a very small wire, like #36 or so, presumably for
low capacitance. It was broken. I replaced it with #24 wire.

[Steve McVoy]

Those voltages are absolute - not relative to the cathode. They are from the factory manual schematic.

I'm sure you have checked the 7500v line?

Focus voltage won't be precisely 2000, and in some cases I've had to change the divider to get a voltage that will produce a focused picture.

[dtvmcdonald]

You are right about the 2000v focus ... its very wrong. The resistor
chain values on the schematic give more like 1400 and result in
correct focus near the center of the control.

I changed all the resistors to modern carbon ones, using two or three
2-watt ones in series for each of the originals to get enough voltage
rating.

I have no idea if this helped, however.


That's because I discovered that the focus wire to the
original socket was open. It was NOT OPEN yesterday morning, because I
measured it when making the adapter for the radar tube. But it
sure is now. How can a wire go open? This is not a small gauge wire,
its like #22 stranded. I did not cut open the bundle it is in but replaced
it with a new HV wire cable-tied to the bundle.

I did cut about 1/2 inch off it at the socket end and 2 inches inside
the chassis. I stripped the ends expecting to find that the shortened
wire was OK. No, its open. How can such a wire be open?

In any case the set now produces a quite nice picture. Its not without
defects. First is what looks like interference but probably
is beats between the audio and color carriers. I've got to figure
out where the audio trap is and check its adjustment.

Second is blurring due to too much capacitance from the grid lead.
This varies with placement. I will make new cables with 0.005 inch
wire, and also make spacers to keep it away from the rest
of the bundles.

Other than that, the set now works correctly.

[dtvmcdonald]

Part of the blurring and the interference lines were only on ch.4, not 3. The lines
indeed were audio-color beating.

The cause was a misadjusted RF coil in the tuner. Now both 3 and 4 produce nice pictures.

[Steve McVoy]

Great news. Congratulations on getting it working.

I'm sure you know that with the low IF frequencies this set has horrible image rejection. I never attempt to feed them with anything but a single channel (3 or 4).

[dtvmcdonald]

I have a rubber mask for this thing that came with the cabinet.

However, this morning I tried fitting it in the cabinet and on the tube
and it appears to have shrunk. It would seriously restrict the
view compared to the area of the wood mask. Is shrinkage normal
or at least precedented?

If it is not normal, and no proper replacements I exist I can of course
kludge something out of foam.

[Steve D.]

This ad on the ETF classifieds page:
Reproduction TRK-12 rubber CRT masks, $100 a piece. Contact Barry Kasindorf, barry at barrykasindorf dot com 508-528- 0026.

Don't know if it is still current

-Steve D.

[Steve McVoy]

Depends on what you mean by shrinking. Attached are a couple of photos that show what a good one looks like.

[dtvmcdonald]

I got all the parts in the cabinet safely and it works. The mask is indeed
the right size.

All that's left is a permanent bucking transformer, which I have plenty of.
I'm currently using a variac.

The only problem I had in putting it in the cabinet is that the
picture was rotated 180 degrees. I didn't feel like removing the tube to rotate the yoke
so I reversed the leads instead. The la
st 1/4 inch of the leads were somewhat dicey
anyway so this had an added bonus of safety.

Another TRK12 lives.